1. Field of the Invention
The present invention relates to a light modulation device and a filter device, and more particularly to a light modulation device and a filter device using a magnetic garnet single crystal.
2. Description of the Related Art
Conventionally, there have been attempts to modulate light using microwaves in a magnetic garnet single crystal such as a YIG crystal (M. Tutumi et al., Technical Report of IEICE MW98-41; C. S. Tsai et al., Appl. Phys. Lett. 47 (7) 651 (1985); A. D. Ficher et al., Appl. Phys. Lett. 41 (9) 779 (1982)). The magnetic garnet single crystal (YIG) used in these light modulation devices has generally been a bulk single crystal or an epitaxial thin-film. In this case, there has been no preferred direction of the magnetic garnet single crystal, but in many cases where an epitaxial thin-film is used, the face direction is selected to be a  less than 111 greater than  direction.
In a conventional microwave light modulation device using a magnetic garnet single crystal, greater modulation efficiency can be achieved using a thick bulk single crystal as the magnetic garnet single crystal, rather than by using a thin epitaxial thin-film.
However, although a superior large-scale epitaxial thin-film can be obtained comparatively easily and with few defects using an LPE method, or sputtering, or the like, it is difficult to obtain a bulk single crystal by these methods.
Accordingly, it is an object of the present invention to provide a light modulation device and a filter device which have high modulation efficiency using either a bulk single crystal or an epitaxial thin-film as the magnetic garnet single crystal.
The light modulation device may comprise: a magnetic garnet single crystal having a main surface and an easy magnetization axis in parallel with the main surface; an optical source arranged such that light emitted from the optical source is introduced into the magnetic garnet single crystal and propagates in the magnetic garnet single crystal along a first direction on the main surface; and one or a pair of transducers provided on the main surface of the magnetic garnet single crystal for carrying a microwave signal in a region of the magnetic garnet single crystal where the light from the optical source propagates, so as to modulate the light.
The magnetic garnet single crystal preferably has a  less than 110 greater than  direction or a  less than 211 greater than  direction as the main surface. The first direction on the main surface of the magnetic garnet crystal and the easy magnetization axis are preferably parallel with each other. Moreover, a magnetic field is preferably applied to the magnetic garnet single crystal along the first direction.
The magnetic garnet single crystal may comprise Y3Fe5O12, or alternatively at least one of the Y and the Fe of the Y3Fe5O12 may be replaced by another element. The magnetic garnet single crystal may be a bulk single crystal. The magnetic garnet single crystal may also be an epitaxial thin-film, or an epitaxial thin-film made using a liquid phase epitaxial method.
According to another embodiment of the present invention, a filter device comprises: a light modulation device as described herein; and a photodetector for detecting the light propagated in the magnetic garnet single crystal.
According to the present invention, a light modulation device and a filter device can have high modulation efficiency, whether using a bulk single crystal or an epitaxial thin-film as the magnetic garnet single crystal.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.